The present invention relates to using backup or spare communications circuitry in a communications system as needed, and particularly relates to using separate communications signal paths in circuit sparing operations.
Modern telecommunication systems are extraordinarily complicated, and yet they are oftentimes taken for granted by the consumers who use them. This tendency for people to under appreciate their underlying complexity is perhaps the greatest testament to the reliability of modern telecommunication systems. Achieving this level of reliability in a complex network of interconnected equipment requires substantial effort, ingenuity, and a strong belief in contingency planning.
Failure at some point is inevitable for all electronic circuits. One may assume that any electronic circuit is subject to failure at some time. This basic assumption finds its fullest refinement in the art of reliability engineering, where complex prediction techniques allow engineers to estimate values for parameters such as mean-time-to-failure (MTTF) for various circuits and systems. While the use of reliable components and careful engineering results in telecommunication circuits with low rates of failure, the possibility of failure is inescapable. Some telecommunication circuits are simply too critical to leave this possibility of failure unmitigated. In such cases, it is common to provide spare or backup communication circuits that are used to substitute for primary circuits as needed.
In 1:1 sparing schemes, each critical primary circuit has a corresponding spare circuit. However because of constraints on equipment size and cost, 1:1 sparing is reserved for the most critical circuits. As an alternative, a piece of telecommunications equipment may include a single spare circuit that may be substituted for any one of a number of primary communication circuits. The sparing of this type is referred to as N:1 sparing. A common problem with existing sparing techniques is that the communication signal paths running through the primary circuits are also routed through the sparing circuits. For example, switches or their electronic equivalents are commonly used to connect communication lines to either primary communications circuits or to a spare communications circuit. While this does allow switching between primary and backup communications circuits as needed, it makes the primary communications signal paths directly dependent upon the switches themselves. Thus, switch failures or switch maintenance activities undesirably disrupt communication signals from the primary communications circuits.
Conventional sparing techniques are particularly problematic in telecommunications line interface equipment. In a high-density system, the telecommunications line interface equipment uses a number of line cards to interface a large number of telecommunication lines with associated transmission or network equipment. Each line card carries interface circuits for a substantial number of telecommunication lines. Because of the large number of telecommunication lines serviced by any one line card, removing even a single line card from service would result in a loss of service to a greater number of customers than controlling telecommunications standards allow. With conventional sparing techniques applied to an N:1 arrangement, each of the telecommunication lines is routed through some type of switching circuitry that selectively switches the telecommunication lines from any primary line card to a spare line card. While this solves the problem of providing a spare line card when needed, it also means that any problems or maintenance associated with the switching circuitry can remove at least as many telecommunication lines from service as would the failure of one of the primary line cards.
Accordingly, there remains a need for a telecommunications line interface architecture that includes sparing circuitry that may be used as a backup to one or more primary communication circuits, without failures or maintenance of the sparing circuitry compromising the availability of the primary communication circuits.
The present invention splits telecommunication lines, such as DS1 subscriber lines, into primary and secondary signal paths within a piece of telecommunications line interface equipment. The primary signal paths connect the telecommunication lines to primary interface circuits, while the secondary signal paths are used to selectively connect desired ones of the telecommunication lines to spare interface circuits as needed. Preferably, the present invention includes a switching network that operates only on the secondary signal paths to connect selected ones of the telecommunication lines to the spare interface circuits. As such, the telecommunication lines remain connected to the primary line interfaces through the primary signal paths, which are independent of the switched secondary signal paths used with the spare line interface circuits. This allows maintenance to be performed on the switching network or on the spare line interface circuits without disrupting services on any of the telecommunication lines supported by the primary interface circuits.
Preferably, the present invention includes circuitry for splitting incoming telecommunication lines into corresponding primary and secondary lines, and operating on the secondary lines to implement sparing functions. The primary lines couple directly to the primary line interfaces through the primary signal paths and are not routed through the sparing circuitry operating on the secondary lines. The sparing circuitry may include sparing switches (switching network) and associated control circuitry for selectively connecting desired ones of the secondary signal paths to a spare line interface. Preferably, the sparing circuitry is configured as a backplane, sparing switch cards, and a sparing switch control card. The backplane connects a group of secondary signals to each sparing switch card, and each sparing switch card includes switched connections for selectively connecting its associated group of secondary signals to a sparing bus carried on the backplane. A sparing bus interface on the backplane allows connection of the sparing bus with the spare line interface. When a given primary line interface fails or requires maintenance, the telecommunications line interface equipment uses the sparing circuitry of the present invention to connect the group of secondary lines corresponding to the primary lines associated with the given primary line interface to the spare line interface by controlling the appropriate switches on the sparing switch cards.
Typical telecommunications line interface equipment organizes line interface circuitry into one or more line cards. In high-density systems, a single line card may interface with a substantial number of telecommunications lines. Telecommunication standards dictate a maximum number of such lines that may be out of service at any one time due to maintenance activities or circuit failure. Thus, even the loss of one such line card may represent an unacceptable loss of service. The present invention may be advantageously used in such a system to provide sparing functions for any one of the primary line cards using a single backup line card. Unlike conventional approaches to sparing critical telecommunication circuits, the present invention does not use a common signal path for both sparing functions and primary communications. With a common signal path, failures or maintenance of sparing-related circuits disrupts primary communications, which is unacceptable in the high-density systems described above. The split secondary and primary signal paths provided by the present invention allow sparing functions to use secondary signal paths that do not interfere with primary communications. Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of exemplary embodiments of the invention in conjunction with the accompanying figures.